1. Field of the Invention
The present invention relates generally to communication systems, and in particular, to an apparatus and method for adaptively controlling a channel data rate according to a channel environment in a CDMA communication system.
2. Description of the Related Art
At present, CDMA (Code Division Multiple Access) communication systems are implemented in accordance with the IS-95 Standard. With the progress of mobile communication technology, the number of mobile communication service subscribers is increasing and demands for various services are rising in proportion to the increased user demand. To date many methods have been proposed to meet the subscribers' demands.
FIG. 1 illustrates a structure of a forward traffic channel transmission device for the CDMA communication system, wherein the traffic channel includes a fundamental channel and a supplemental channel.
Referring to FIG. 1, a channel encoder and puncturing part 10 encodes and punctures input data and outputs symbol data. A convolutional encoder or a turbo encoder can be used for the channel encoder and puncturing part 10. A symbol repetition part 20 repeats the respective encoded symbol data for the input data having different bit rates to output a single common symbol rate. An interleaver 30 interleaves an output of the symbol repetition part 20. A block interleaver can be used for the interleaver 30.
A long code generator 91 generates long codes for the user identification, which are uniquely assigned to the respective subscribers. A decimator 92 decimates the long codes so as to match a rate of the long codes to a rate of the symbols output from the interleaver 30. A mixer 40 mixes the encoded symbols output from the interleaver 30 with the long codes output from the decimator 92.
A signal mapping part 50 maps binary data output from the mixer 40 into 4-level data by converting data “0” to “+1” and data “1” to “−1”. An orthogonal modulator 60 modulates data output from the signal mapping part 50 with an orthogonal code. A Walsh code can be used for the orthogonal code. In this case, Walsh codes of lengths 64, 128 and 256 bits can be used. A spreader 70 spreads the orthogonal modulation signal output from the orthogonal modulator 60 by combining it with spreading sequences. PN (Pseudo-random Noise) sequences can be used for the spreading sequences. Accordingly, a QPSK (Quadrature Phase Shift Keying) spreader can be used for the spreader 70. A gain controller 80 controls a gain of the spread signal input from the spreader 70 according to a gain control signal Gc.
In operation, when the convolutional encoder is used for the channel encoder and puncturing part 10, the coding rate is ⅓ and the constraint length, k=9, for an IS-95 system. Therefore, one input data bit is encoded into three encoded bits (i.e., three symbols) in the channel encoder and puncturing part 10 (which performs ⅓ rate convolutional encoding or ⅓ rate forward error correction (FEC)). Forward error correction is utilized to provide coding gain to a channel so as to compensate for an increase in a BER (Bit Error Rate) at a mobile station (for the case of a forward link) and a base station (for the case of a reverse link). An increase in the BER of a channel may arise as a result of the channel having a reduced SNR (Signal-to-Noise Ratio) due to an increase in signal path loss, noise and interference.
It is well known that CDMA communication systems cannot provide reliable communication service when a mobile station is located at an outer service area of the base station or is in a bad channel environment. In this case, it is preferable to change the coding rate to enhance the quality of the communication service in the bad channel environment. That is, when the channel SNR is reduced due to a bad channel environment or an increased distance between a mobile station and a base station, it is preferable to use a coding rate (or FEC rate) lower, ⅙ for example, than the present coding rate of ⅓.
In particular, when the distance between the base station and the mobile station increases, a reception device is very susceptible to path loss or noise on the link channel and interference, so that the channel SNR is reduced unless a transmission device increases the transmission power or performs a pertinent compensation. Therefore, when the traffic channel transmission device with the fixed channel structure of FIG. 1 experiences an increased BER (Bit Error Rate) due to a reduction in SNR, the base station increases a forward link traffic power in order to compensate for the increase in the BER. Therefore, it is preferable to use the FEC with a lower coding rate than the FEC in use. Given a ⅓ coding rate it has been shown that channel gain is lower by about 0.2–1 dB as compared with a ⅙ coding rate. For example, the forward reception power of a mobile station using the ⅓ coding rate is lower by about 1 dB than that of a mobile station using the ⅙ coding rate, when the mobile station is far from the base station or in a bad forward channel environment. Therefore, the base station should increase the forward link transmission power, resulting in a waste of transmission power and low communication performance.
Unlike the channel transmission device with the fixed channel structure of FIG. 1, a channel transmission/reception device for a 3rd generation multicarrier CDMA system as proposed in the TIA/EIA TR45.5 conference, includes a scheme for transmitting and receiving the respective channel data by distributing them to the multicarrier. For example, when three carriers are used and a rate ⅓ encoder is used, the multicarrier scheme encodes the respective input data bit into three encoded bits (i.e., symbols) using the rate ⅓ encoder and transmits the encoded bits using the three carriers after repetition and interleaving. This is well disclosed in Korean patent application No. 61616/1997 filed by the applicant of this invention. Here, the respective carriers each have a bandwidth of 1.2288 Mhz (hereinafter, referred to as 1.25 Mhz) which is identical to the IS-95 channel bandwidth. Therefore, the three carriers have a combined or collective bandwidth of 3.6864 Mhz, which is identical to three separate channel bandwidths.
The forward link of the 3 G multicarrier system can employ an overlay method where it shares the same frequency band with the IS-95 forward channel. In this case, it may be interfered with the IS-95 system. In addition, it is preferable to use the coding rate lower than the present coding rate of ⅓, even when the channel SNR is reduced due to the bad channel environment or the increased distance between the mobile station and the base station.